Falling regulatory limit values for emissions of combustion engines, particularly of diesel engines, necessitate active emissions-reduction methods as well as methods for measuring low particle concentrations in the exhaust gas flow.
Particles from diesel engines may include a significant portion of soot and hydrocarbons absorbed on it. By optimizing the combustion processes, it has been possible to reduce the emitted particle mass. In particular, the number of large particles having diameters of more than one micrometer, which formerly contributed to the dark coloration of diesel emissions, has been drastically reduced.
Because of the low mass concentration and the small average diameter of the particles, the demands on measuring methods for determining the particle mass limited by law is increasing. This may be true in particular for particle sensors used in motor vehicles for regulating emissions-reduction components and/or the engine. In addition to sufficient accuracy and response time, suitable sensors may also be subject to requirements with respect to mechanical and thermal stresses, and at the same time, to requirements that they be manufactured in large quantities at low cost.
Sensors are already available for stationary engine test benches which, for example, may be based on the measurement of the blackening number, the opaqueness, or the gravimetric measurement of the change in the weight of a filter. These and other measuring methods may only partially satisfy the requirements indicated above.